1. Field of the Invention
The invention relates to a torque measuring device which measures torque of a rotating body, more particularly to a device for measuring torque of a rotating body, in which electricity and measurement signals are transmitted and received in a noncontact manner between a rotary section and a stationary section in order to measure torque of the rotating body with a strain gage, and to a method for measuring the torque.
2. Description of Related Art
Conventionally, a device, in which a plurality of strain gages connected so as to constitute a Wheatstone bridge circuit are attached by means of adhesive to a cylindrical distortion generating body connected between a rotating shaft and a load shaft driven by the rotating shaft and in which thereby a torsion quantity generated in the distortion generating body is converted into electricity quantity as torque quantity of a rotating body and transmitted to the stationary section, has been used for measurement of the torque of the rotating body. In this measuring device, important are a method for transmitting required electric power from a stationary section to a circuit on a rotary section and a method for transmitting to the stationary section an electrical signal gained by converting a torque quantity of the rotary section into electricity quantity where various problems have been pointed out in conventional mechanical contact methods.
In order to solve the above-mentioned problems with the mechanical methods, a method for converting an electrical signal of a rotating body into an optical signal and transmitting the optical signal to a stationary section: xe2x80x9cDetection Signal Transmission Device for Physical Quantity-Electrical Quantity Converter for Rotating Bodyxe2x80x9d has been proposed (Japanese Patent Laid-open No. Hei 6-301881). Hereinafter, measuring mechanism of the conventional xe2x80x9cDetection Signal Transmission Device for Physical Quantity-Electrical Quantity Converter for Rotating Bodyxe2x80x9d will be described based on FIG. 7.
The measuring mechanism comprises five main parts including a torque transmission part, a photo coupling part, a torque detecting part, signal processing circuit part and electricity receiving part. The main parts relating to the invention will be explained as follows.
In FIG. 7, the torque transmission part is constituted as a so-called flexible coupling mechanism providing a connecting function with flexibility. The main mechanism thereof is formed symmetrically with respect to a distortion generating body 701.
The photo coupling part is a mechanism which transmits a measured signal from a rotary section to a stationary section. A signal transmission part on the rotary section is constituted by a light emitting side ring 714 attached to the distortion generating body 701 and a plurality of light emitting diodes 721 which are electricity-into-light conversion elements and attached to the light emitting side ring 714.
Moreover, a signal receiving part on the stationary section is constituted by a light receiving side ring 713 and a plurality of photodiodes 720 which are light-into-electricity conversion elements and attached to the light receiving side ring 713. The light emitting side ring 714 of the signal transmission part is formed such that its inner periphery is fixed to the distortion generating body 701, and its outer periphery is shaped substantially bowl-like in cross-section. The light emitting diodes 721 are attached to a bottom part of the bowl shape.
On the other hand, the ring 713 of the signal receiving part is formed such that its inner periphery is shaped substantially bowl-like, and the photodiode 720 are attached to a bottom part of the bowl shape. These two kinds of rings, namely the light receiving side ring 713 and the light emitting side ring 714 are arranged so that the substantially bowl-shaped opening portions face each other and are positioned close to each other. Eight pieces of the photodiodes 720 are attached to the inner periphery of the light receiving ring 713. And, five pieces of the light emitting diodes 721 are attached along the entire periphery of the ring and are simultaneously lightened.
The distortion generating body 701 is a cylinder with a large diameter as a whole, where a distortion generating part 701a has a reduced thickness at its center area in the axial direction, and in the area with the reduced thickness, two side surfaces symmetrical with respect to the axis of the cylinder are shaved into flat surfaces. Strain gages 723 are attached to the flat portions and are incorporated into a Wheatstone bridge circuit 723 having two attached sheets (a pair) of the strain gages 723 as opposing sides.
The signal processing circuit 722 includes a circuit where a detection signal detected by the strain gages 723 attached to the distortion generating part 701a as the torque detecting part is appropriately amplified, waveform-shaped and signal-processed and where the processed signal is outputted to the photo coupling part comprising the light emitting diodes 721 and the photodiodes 720.
The electricity receiving part is constituted by a noncontact-type rotary transformer. An electricity transmitting ring 711 on the stationary section and an electricity receiving ring 712 on the rotary section are arranged facing each other and close to each other. At opposing side surfaces of the rings 711,712, a pair of substantially U-shaped ferrite cores (not shown) are mounted with their openings facing each other. An electricity transmitting coil 716 and an electricity receiving coil 717 are wound in a ring-like manner within respective spaces inside the substantially U-shaped ferrite cores.
In the conventional method for converting an electrical signal of the rotary section into an optical signal and transmitting the optical signal to the stationary section, there has been such a problem that a plurality of electricity-into-light conversion elements are arranged on the rotary section leading to an increased power consumption on the rotary section and therefore the size of a rotary transformer which transmits electric power from the stationary section to the rotary section has to be increased. There has been another problem that the rotary transformer for transmitting power and a light conversion part for transmitting a torque signal from the rotary section to the stationary section are separately structured, with the result that the device had to be increased in size and that it was difficult to properly position each part.
Moreover, since there were eight pieces of light receiving rings mounted at equal intervals and five pieces of light emitting diodes mounted at equal intervals throughout the entire ring and so on, making the manufacturing difficult and it was required to select elements which have small variance in characteristics from each other and which have a wide range (for example xc2x1100 degrees) of directivity characteristics to uniformly transmit light.
Further, unless the five pieces of light emitting diodes are lightened simultaneously, the light emitting region is not shaped in an arch band of about 45xc2x0, forcing control of a drive circuit to be complicated. And, in case one or more of the light emitting diodes or the light receiving diodes fail to duly function, torque can not be detected at some places causing a problem.
Furthermore, the distortion generating body is a cylinder with a large diameter as a whole, but the distortion generating part which is a torque detecting part has a reduced thickness at its center area in the axial direction, and in the area with the reduced thickness, two side surfaces symmetrical with respect to the axis of the cylinder are shaved into flat surfaces, on which stress is concentrated causing a strength problem.
It is an object of the invention to provide a device having a structure in which electric power is efficiently transmitted from a stationary section to a rotary section rotating at a high speed and a circuit in which a signal of an electrical quantity converted from a torque quantity detected at the rotary section is transmitted to the stationary section without deteriorating the S/N ratio.
Accordingly, it is an object of the present invention to provide a torque measuring device to measure rotating torque and at the same time to obtain a method for measuring torque of a rotating body with this torque measuring device in order to solve the above mentioned problems.
In order to achieve the above mentioned object, a torque measuring device of the invention comprises a rotary section including a driving flange and a driven flange on both ends thereof; a half-blindly hollowed cylinder having a circumferential wall configured in section to form a U-defined open space between the flanges and having a prescribed thickness and a prescribed diameter at a very bottom of the open space; a torque detecting means disposed on an inner circumferential surface of the cylinder and adapted to convert a physical quantity corresponding to a value of torque acting on the cylinder portion into an electrical signal; a stationary section opposing an outer circumference of the driven flange; a set of rotary transformers comprising a first transformer core which is formed on the outer circumference of the driven flange and has a first groove and a second transformer core which is formed on an inner circumference of the stationary section and has a second groove; an optical signal conversion means composed of at least one electricity-into-light conversion element, disposed in the first groove and adapted to emit light according to an output of the torque detecting means; a light transmission means composed of an optical fiber, disposed in the second groove and adapted to receive the light from the optical signal conversion means; and a light detecting means to detect the light transmitted from the light transmission means.
Accordingly, the rotary transformer in which stress is not undesirably concentrated on a thin portion, strength of the thin portion is not lowered and in which a high torsional rigidity for high response to a sudden torque change is present can be structured with a compact dimension. Moreover, since the number of the conversion elements can be reduced, it is possible to reduce power consumption both on the stationary section and the rotary section and to significantly increase average time between failures, which leads to improvement of reliability.
In the torque measuring device, the light detecting means may comprise a light-into-electricity conversion element provided on at least one end surface of the optical fiber. As a result, the number of the parts is significantly reduced.
In the torque measuring device, the light detecting means may be disposed, together with the light transmission means, in the second groove formed on the second transformer core which opposes the driven flange and completes the set of rotary transformers with the first transformer core. According to this embodiment, it is possible to eliminate visible light leaking from the proximity of the connecting point of the light transmission means and the light detecting means.
In the torque measuring device, the optical signal conversion means provided in the first groove and the light transmission means and the light detecting means both provided in the second groove may be disposed so as not to protrude beyond an interface defined by the first and second transformer cores. Therefore, efficient transmission and receipt of electric power can be carried without lowering efficiency of the rotary transformers.
In the torque measuring device, the light transmission means and the light detecting means both provided in the second groove may be covered with a visible light cutoff filter. Accordingly, high S/N ratio can be obtained.
In the torque measuring device, the light transmission means may be set in such a way as to be wound in at least one turn within and along the second groove. As a result, torque is detectable anywhere regardless of the position of the rotating body.
And a method for measuring torque of the present invention comprises the steps of:
setting a torque detecting means, which converts a physical quantity corresponding to a torque value into an electrical signal, on an inner circumferential surface of a cylinder portion at a place where the cylinder portion has a smallest thickness in its circumferential wall between a driving flange and a driven flange;
converting the electrical signal from the torque detecting means into an optical signal by an electricity-into-light signal conversion means and transmitting the optical signal to a stationary section;
directing the optical signal transmitted from a rotary section onto a side face of an optical fiber provided in a groove formed on a transformer core which is provided in the stationary section in such a manner as to oppose the driven flange, thereby converting the optical signal into an electrical signal;
subjecting the electrical signal to a prescribed waveform-shaping process for a frequency modulated signal;
demodulating the frequency modulated signal; and retrieving the demodulated signal as an analogue signal by way of a prescribed circuit.
According to the method, it is possible to obtain a structure to efficiently transmit electric power from the stationary section to the rotary section rotating at a high speed and measure torque without deteriorating the S/N ratio of the signal converted from the torque quantity detected on the rotary section into an electrical quantity.
In the invention, a sensor with a strain gage is disposed on the inner circumferential surface of the hollowed cylinder having the circumferential wall configured in section to form a U-defined open space between the driving flange to be fixed to a rotating body and the driven flange and having a prescribed thickness at the very bottom of the open space. The sensor converts a physical quantity corresponding to a value of torque acting on the hollowed cylinder into an electrical signal. The electrical signal is further converted into an optical signal. Then, by the optical signal conversion means composed of at least one electricity-into-light conversion element, disposed in the first groove and adapted to emit light according to an output of the torque detecting means, the optical signal is made incident onto the side face of the optical fiber provided in the second groove formed on the second transformer core which is provided in the stationary section in such a manner as to oppose the driven flange. The optical signal made incident onto the side face of the optical fiber is then converted into an electrical signal by the light detecting means adapted to receive light reaching the end face of the optical fiber, whereby a torque value measured is obtained.